1. Field of the Invention
This invention relates to a ceramic rotor of unitary structure and a manufacturing process therefor, which ceramic rotor has a blade-holding portion with excellent mechanical properties at high temperatures and blade portions with excellent heat resistivity, the blade portions being integrally cemented to the blade-holding portion by sintering.
2. Description of the Prior Art
In recent years, to meet the need of energy saving, research efforts have been made to improve the efficiency of turbines and engines by raising the running temperature thereof. In order to run the turbines and engines at a temperature higher than 1,100.degree. C., the engine rotor or turbine rotor is required to have excellent heat resistivity. Furthermore, turbine rotors rotate at high speeds such as 50,000 to 160,000 RPM (revolutions per minute) and experience high tensile stress at high temperatures, so that large forces are applied especially to blade-holding portions of the rotors. Thus, the material of the rotors for high-temperature running is required to have excellent high-temperature strength. Conventionally, nickel- or cobalt-base heat-resisting metals have been used to make heat-resisting turbine rotors, but such conventional heat-resisting metals hardly endure temperatures in excess of 1,100.degree. C. for a long period of time and are very expensive due to limited resources of nickel and cobalt. To replace the expensive heat-resisting metals, ceramic materials with excellent high-temperature properties such as silicon nitride (Si.sub.3 N.sub.4), sialon, and silicon carbide (SiC) have been studied. For instance, turbine rotors are made of ceramic materials by either one of the following three methods.
(1) A blade-holdig portion having grooves on the outer surface thereof is made of silicon nitride (Si.sub.3 N.sub.4) by hot pressing. Blade portions of complicated three-dimensional shape are formed by injection molding of silicon (Si) powder, and the thus formed blade portions are transformed into silicon nitride (Si.sub.3 N.sub.4) blade portions of reaction-sintered type by nitriding and sintering. The silicon nitride (Si.sub.3 N.sub.4) blade portions of reaction-sintered type are fitted in the grooves of the silicon nitride (Si.sub.3 N.sub.4) blade-holding portion one by one, and the blade portions and the blade-holding portion are integrally cemented by either hot pressing or hot isostatic pressing.
(2) Metallic silicon is poured between those surfaces of the aforesaid grooves of the silicon nitride (Si.sub.3 N.sub.4) blade-holding portion made by hot pressing and the aforesaid silicon nitride (Si.sub.3 N.sub.4) blade portions of reaction-sintered type which are to be cemented, so that intermediate layers are formed between the surfaces of the blade portions and the blade-holding portion being cemented. Then, the thus assembled blade portions, blade-holding portion, and the intermediate layers are subjected to nitriding and sintering, so that the metallic silicon of the intermediate layers is transformed into silicon nitride (Si.sub.3 N.sub.4) and the blade portions are integrally cemented to the blade-holding portion.
(3) Blade portions and a blade-holding portion are integrally formed by injection molding of a ceramic material such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), or metallic silicon (Si). The integral body thus molded is sintered either in an inert gas atmosphere in the case of silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (SiC), or in a nitrogen gas atmosphere in the case of the metallic silicon (Si).
However, the three methods have shortcomings; namely, the method (1) is not suitable for mass production and very costly because special facilities for the hot pressing are required to produce the blade-holding portion and to cement the blade portions to the blade-holding portion; the method (2) is costly due to the need of hot pressing in producing the blade-holding portion and tends to result in insufficient strength of the cemented surfaces due to the cementing of different materials, i.e., hot pressed silicon nitride and reaction-sintered silicon nitride; and in the method (3), it is difficult to make dense and very strong blade-holding portions and when there is a large difference in thickness between the blade portions and the blade-holding portion, cracks are caused during binder removal treatment so that the yield is low and the method is costly in consequence.